Many paper applications require not only high strength but also functionalities that provide the paper article with moisture, oil and grease, mold and fire resistance, increased brightness, or other specialized functionalities like antimicrobial properties or magnetic properties. Certain of these products are currently manufactured by imparting paper a coating in a secondary process. In one approach for adding functionality to the paper surface, the sizing process uses cooked starch solutions with additives (such as brightening agents, clays, hydrophobicizing compounds) to impart surface functionality to the paper. In the sizing process, the wet web is first dried to a pre-set moisture content and/or is re-wet to achieve uniform moisture content throughout; then the material is fed into a size press where a high loading of gelatinized starch with additives is applied to the paper surface; then the material is dried again. This process involves a number of downstream processes that can be inefficient. Inefficiencies result from the number of steps involved in preparing the substrate, cooking the starch and applying it to form the finished product. A considerable amount of energy is required for these steps, which adds to the costs of the process.
For certain paper products, functionalities can be added by incorporating additives into the fibrous matrix during the papermaking process. Particulate additives can be introduced into the paper web, substituting for some of the pulp that might be used otherwise. These particulate fillers can create, for example, a bulky final paper product that creates the impression of higher quality through its tactile properties while minimizing the use of expensive pulp. Particulate fillers can also be used to impart other specialized properties besides bulk. For example, particulate additives can include filler particles, or other particles, suitable for use papermaking or a final paper product can include mineral particles such as calcium carbonate, dolomite, calcium sulfate, kaolin, talc, titanium dioxide, silica, aluminum hydroxide, and the like. Particles can be formed from inorganic or organic materials, and may be solid or porous. Organic particles may be polymeric, optionally crosslinked, and may be elastomeric. A wide variety of particles known in the art can be incorporated into the finished paper product to improve performance attributes such as brightness, opacity, smoothness, ink receptivity, fire retardance, water resistance, bulk, and the like.
Precipitated Calcium Carbonate (PCC) is particularly useful as a particulate filler additive where high opacity, brightness and maintenance of caliper are required. Higher PCC contents replace expensive pulp improving the profitability of paper. Although PCC contents as high as 15% are often used in papermaking, the first pass retention of the filler is poor, so that a significant amount can be lost from the paper product during the papermaking process. The PCC that is incorporated into the paper product also leads to weaker sheets, because the particles themselves disrupt the hydrogen bonding between cellulose fibers. Higher ash content (>15%) is highly desired in the paper industry, where ash content indicates the amount of filler in a paper.
In another embodiment, TiO2 particles are highly desired as particulate fillers to improve the opacity and brightness beyond what is achievable using PCC. The TiO2 particles due to their small size and high refractive index are capable of scattering light and improving the opacity of the paper containing them. As the TiO2 particles are many times more expensive than PCC, improvement in retention is highly desired. Although flocculants can be used to improve the retention of TiO2, the flocculated TiO2 particles do not possess the same optical properties as the individual TiO2 platelets. It would be advantageous to combine TiO2 particles with other particles to form a composite that separates individual TiO2 particles and allows them to retain their optical characteristics.
Other particulate fillers can be added to the paper product to impart specific, desirable properties. As an example, magnetic or paramagnetic particles can be incorporated into the paper to form a magnetic or a magnetizable paper. As another example, colloidal silver particles can be introduced into a paper product to impart antimicrobial properties. A large number of additives can be contemplated that are available in particulate form, including additives that impart oil or grease resistance, optical brightening, ink binding, dust control, water repellency, stiffness, biocidal properties, bioactive properties (e.g., a biomolecule for controlled release), adhesive properties, diagnostic sensing, filtration assist, targeted capture/sequestration, and the like. For particulate additives, proper distribution within the paper matrix is important. For particulate additives that are expensive, proper retention is also important. And with the addition of any additive, its impact on the strength, stiffness and bulk of the final paper product must be considered.
A variety of other additives can be used to impart desirable properties to paper products, but face some of the same challenges: retention, distribution and impact on paper quality. Some other additives used presently to impart various functionalities to paper include synthetic fibers (imparting strength and hydrophobicity and absorbency characteristics), latex colloids (imparting properties such as hydrophobicity, oil and grease resistance, mold resistance, fire retardancy, impact resistance), etc. These components have poor affinity to pulp fibers, though, owing to lack of functional groups capable of interacting with cellulose fibers. As an example, latex colloids are particularly useful for imparting resilience, barrier properties, bulk, impact resistance, damping, and the like. Latex particles that are micron or submicron sized (typically 100 nm particles) suspended in an aqueous solution are particularly suited for use in papermaking. However, latex is typically water-insoluble, and can be integrated only with great difficulty into an aqueous process like papermaking.
It is desirable, therefore, to have a process where an additive capable of delivering added functionality can be mixed with pulp fibers in the wet-end of papermaking such that the additive becomes an integral part of it. It is desirable that such additives be distributed evenly and appropriately within the paper matrix, and that the additives be retained on the product and not lost in the whitewater. It is further desirable to introduce such additives so that they preserve the strength and resiliency of the final paper product.
As an example, there exists a particular need in the art for systems and methods that incorporate and retain colloidal latex particles in the wet end so that high amounts of these fillers are dispersed uniformly in the paper providing paper with desired functionalities. These colloidal latex fillers should, desirably, be incorporated so that they are stably anchored to the pulp fibers, allowing them to expand or gelatinize during paper manufacturing without being dislodged. In this manner, the fillers can occupy the interstitial spaces between cellulose fibers more completely, improving the properties of the paper product. Furthermore, it is known that high filler content has a detrimental effect on the strength of the wet web before it is dried because the fillers act as spacers and interfere with fiber-fiber bonding. An efficient retention system that attaches the latex fillers to fibers durably in the wet web can advantageously enhance wet web strength during processing by allowing fiber-fiber bonding to proceed unimpeded.